Roster's study, mentioned above, is pretty old. Steel loads are much better now than they were when he did that study (I think about 20 years ago). And he also used 1 oz loads, assuming that most pheasant hunters would go with the cheap stuff. (And back then, steel was significantly more expensive than lead.) It was a blind study (shooters didn't know what they were shooting), and compared shot sizes 2, 4, and 6.
One interesting result from the study was that there were almost no birds lost with any of those loads inside of 30 yards. Beyond 40 yards, different story. Wounding loss rate on those longer range birds was something like 15%.
Comparing to today's loads is difficult because of the improvements in steel shotshells. I have a friend who shoots nothing but steel at pheasants. He's a decent shot, but not outstanding. Certainly misses from time to time. Uses 1 1/8 oz 4's, shoots dozens of wild roosters in a season, and seldom loses a bird. I would add that he hunts over a very good wirehair.
